1. Field of the Invention
This invention relates generally to structural panels made from composite materials and, in particular, to honeycomb sandwich panels.
2. Description of Related Art
Composite materials are used extensively in the aerospace industry and in other situations where high strength, stiffness and light weight are desired. Monolithic composite structures typically include fibers and polymer resin as the two principal elements. A wide range of fiber types has been used in composites. Glass, graphite, carbon and aramid fiber are common. The fibers can be chopped, randomly oriented, unidirectional in orientation or woven into fabric. The resin matrix is usually a thermoset material that includes resin, curing agents and other performance modifiers. A fibrous reinforcement (sheet, tape, tow, fabric or mat) pre-impregnated with resin is known as a prepreg.
Core structures are used for certain applications. Honeycomb is a popular core material for such panels because of its high strength to weight ratio and resistance to fatigue failures. Honeycomb cores are made from a wide variety of materials with aramid paper and aluminum being the most common. Prepreg facesheets are bonded to each side of the core.
Both monolithic and core composite structures have to be capable of maintaining an adequate mechanical performance after exposure to wet and moist environments. Honeycomb sandwich panels are particularly prone to moisture ingression into the cells of the core during service.
Technical reports on this subject include “Moisture Ingression in Honeycomb Core Sandwich Panels” by Cise et. al. in the Journal of Materials Engineering and Performance, Volume 6(6), 732, December 1997; “Caution, Honeycomb Core Can Be Dangerous To Your Program's Health” by Campbell in Corrosion Reviews, 25 (1-2), 13-26, 2007 and Loken et. al. in “Water Ingression Resistant Thin Faced Honeycomb Cored Composite Systems with Facesheets Reinforced with Kevlar® Aramid Fiber and Kevlar with Carbon Fibers” published by E.I. Du Pont de Nemours, Wilmington, Del.